Abstract Since there are no lymphatic vessels in brain, cerebrospinal fluid (CSF), the pseudo-lymphatic system, flows around the brain and eliminates protein macromolecules to blood by many routes. The turnover of CSF is reduced with aging and in Alzheimer?s disease (AD). This contributes to the accumulation of protein macromolecules, especially those that are not transported across the blood brain barrier (BBB), such as tau proteins that are associated with AD. However, it?s unclear whether age and AD differential affect the various CSF drainage routes. We used a new protein macromolecule, gadolinium (Gd)-albumin-FITC, as the contrast agent for MRI, followed by fluorescence analysis of the same molecule to specifically delineate macromolecule clearance via CSF flow. Our preliminary data show differences between CSF distribution pattern using Gd- DTPA (standard contrast agent) and Gd-albumin-FITC, with the latter effectively representing the main CSF flow pathways. The distribution of Gd-albumin-FITC in the olfactory bulb, nasal areas, spinal cord and cervical lymph nodes was reduced in APP/PS1 mice compared to normal mice. Tau, brain derived protein, was distributed from brain to CSF and cleared via the cervical lymphatic pathways. CSF flow increased clearance from brain. We hypothesized that Gd-abumin-FITC will effectively assess changes in CSF macromolecule clearance, and thus, will identify clearance pathways of tau proteins that is susceptible to aging. Three aims are proposed to test this hypothesis in aging APP/PS1 mice (2 and 12 months old). Aim 1. Assess CSF clearance pathways in aging APP/PS1 mice using MRI and fluorescence analyses. Aim 2. Assess tau protein clearance via CSF pathways in aging APP/PS1 mice. Aim 3. Assess the efficacy of CSF drainage on tau clearance in aging APP/PS1 mice. The kinetics of macromolecule distribution and elimination in CSF will be determined by using mainly non-invasive techniques, MRI and fluorescence analysis of the same molecule, Gd-albumin-FITC, and near infra-red (NIR)-tau fluorescence. These in vivo real-time imaging followed by confocal microscopy will delineate the CSF flow pathways, CSF/ISF and ISF/CSF exchange and tau clearance pathways. Maps of 3D atlases will identify age/AD-dependent rate limiting pathways. APP/PS1 mice will be used to test the role of amyloid-? in tau clearance in the absence of endogenous human tau oligomerzation. We expect the data will show that tau (monomer>fibril) is cleared by CSF pathways, involving convective flow of ISF, the perivascular space and mainly the cervical lymph, in mice. Aging and A? levels will reduce while increasing CSF flow will enhance tau clearance. Simultaneous studying all the CSF pathways will be unbiased, rigorous and objective in identifying the CSF pathway affected by age/AD, i.e., therapeutic windows, which would lead to novel targets to enhance CSF/tau clearance in the aging brain so as to slow AD related neurocognitive decline and the pending AD tsunami.